But the gallery is also making some expensive lemonade out of lemons. The East Building was going to have to close anyway for building repairs costing $38.4 million. But it secured an additional $30 million private gift to carve out new space, which will mean that when it reopens, visitors will find more exhibition space and a rooftop terrace with stellar views of Pennsylvania Avenue — ideally well before the 2017 inaugural parade. The gallery will spend $68.4 million on both projects.

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A change of profile at the National Gallery of Art

A fair trade-off?

Probably not for fans of the gallery’s growing collection of modern art who won’t be able to view it for three years. But according to gallery officials, repairs make closure of the 35-year-old building essential.

“We did think briefly about the possibility of a kind of staggered closing of the galleries, but that was very hard to do in this open kind of a building. In the end it was just much simpler to close the galleries,” said Harry Cooper, the gallery’s curator and head of modern art.

“This just figures into a long-range planning structure,” said Earl A. Powell, director of the gallery. “There has to be a lot of deconstruction of interior spaces — new elevators, new smoke evacuations. . . . [The building] is seven stories of vertical work and a much more complex building than [the West Building].”

The gallery has known for years that it will need to close its doors to renovate. Since 1999, the gallery has had a multi-phase Master Renovations Plan to update infrastructure in its East and West buildings. The plan includes updates for heating systems, elevators, security systems, fire protection, ventilation and air-conditioning.

But unlike the neoclassical West building, designed by John Russell Pope in 1937, the East Building must close all of its galleries to renovate, a quirk of I.M. Pei’s modern, geometrical design. The galleries in Pope’s building can be cornered off, but the odd, angular spaces in the Pei-designed building make renovations more complicated.

The gallery isn’t the only Washington museum to close for massive renovations in recent memory. In 2000, the Smithsonian announced that the Old Patent Office Building, which at the time housed the National Portrait Gallery and Smithsonian American Art Museum, would undergo a two-year renovation that became a six-year renovation, costing close to $300 million. The renovation was overseen by Hartman-Cox architects, which is also overseeing the National Gallery’s renovation. The Renwick Gallery also announced last month that it will close for two years to renovate, starting next year.

But, as any architect will tell you, closures are the optimal time for ambitious renovations.

“Perry Chin [an associate of Pei’s] said if ‘you’re doing all of that, you might want to think about making new galleries in the two towers,’ ” Powell said. “We could do this, quite easily, while all the light and safety [renovations] closed the building.”

In some ways, the interior gallery addition completes an original design that was never completed. Some of the gallery’s most vocal critics complain that the gallery hasn’t maximized the approximately 120,000 square feet of exhibition space. New galleries will allow for more exhibitions from the permanent collection. And the renovation gives Cooper and his staff time to develop plans for future purchases and exhibitions. The staff will still be working, despite the building’s closure.

“There will be more gallery space and it will be better organized,” Cooper said.

To get a sense of what will happen, one needs to step inside the completed tower, in the southwest pod of the East Building. It has been home to many acclaimed exhibitions — Barnett Newman’s “Stations of the Cross” and Mark Rothko’s “Black Paintings.” But the two towers that face Pennsylvania Avenue were rarely used except for select exhibitions when the ceilings needed to be raised. Those temporary galleries will be gutted and the ceilings will be raised to 23 feet. The hexagonal-shaped towers will be lighted by skylights and connected by an outdoor sculpture terrace, seven stories above street level. Along with sculpture and superb views, the terrace will have park benches, tree planters and a seasonal refreshment area for Washington’s humid summers.

To fund the new gallery space, the gallery approached some notable donors about 18 months ago for the tower gallery renovation, according to Powell. He said the philanthropists — Victoria Sant, the gallery’s president, and Roger Sant, a Smithsonian Regent; Mitchell Rales, a board member, and his wife, Emily; and David Rubenstein, president of the Kennedy Center — each contributed $10 million gifts for the $30 million project.

The East Building has already been through significant maintenance renovations, mostly funded by Congress. Although almost half the age of the West Building, the East building, with its triangular motif, in recent years has required massive facelifts on its facade, with repairs that cost more than $82 million; congressional testimony characterized the problems as “systematic structural failures.”

In 2009, the National Gallery asked for $40 million to repair 16,200 lavender-pink panels of marble from Tennessee that were tilting outward. In 2010, the gallery petitioned Congress for roughly the same amount to complete the facade. Officials expect the infrastructure to be completed by the end of this year, in time for the 2014 closure of the galleries.

This isn’t the end to the building’s renovations. Plans for facilities renovations have been made, but according to Deborah Ziska, spokeswoman for the gallery, those plans are unavailable and “down the road.”

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